The kernel is greppable in either case. It's foolish to rigidly insist that long strings must not be broken. And don't forget that some strings are created dynamically (with %s specifiers, for instance). How are you going to grep for them?

It's also worth pointing out that much of the source code in usbcore tends to indent continuation lines by two extra tab stops. This isn't done all the time, admittedly. But deliberately changing the code so that continuations use only one tab stop feels like a bad idea -- one extra tab stop looks too much like a nested statement.